Integrin αIIbβ3 inhibitors are new class of antithrombotic agents that block fibrinogen binding to the platelet integrin αIIbβ3, thereby inhibiting platelet-platelet interactions essential for the formation of platelet thrombi (Topol, E. J., Byzova, T. V., and Plow, E. F. (1999) Platelet GPIIb-IIIa blockers. Lancet 353, 227-231; Coller, B. S. (1997) Platelet GPIIb/IIIa antagonists: the first anti-integrin receptor therapeutics. J Clin Invest 99, 1467-1471). Integrin αIIbβ3 inhibitors are used for the management of patients with non-ST-segment elevation acute coronary syndromes and patients undergoing percutaneous coronary intervention (Proimos, G. (2001) Platelet aggregation inhibition with glycoprotein IIb-IIIa inhibitors. J Thromb Thrombolysis 11, 99-110). Among these inhibitors, abciximab (ReoPro, Centocor, Inc., Malvern, Pa., and Eli Lilly & Company, Indianapolis, Ind.) (Coller, B. S. (1997) Platelet GPIIb/IIIa antagonists: the first anti-integrin receptor therapeutics. J Clin Invest 99, 1467-1471), eptifibatide (INTEGRILIN, COR Therapeutics, Inc., South San Francisco, Calif., and Key Pharmaceuticals, Inc., Kenilworth, N.J.) (Phillips, D. R., and Scarborough, R. M. (1997) Clinical pharmacology of eptifibatide. Am J Cardiol 80, 11B-20B; Scarborough, R. M. (1999) Development of eptifibatide. Am Heart J138, 1093-1104), and tirofiban (Aggrastat, Merck & Co., Inc., Whitehouse Station, N.J.) (Vickers, S., Theoharides, A. D., Arison, B., Balani, S. K., Cui, D., Duncan, C. A., Ellis, J. D., Gorham, L. M., Polsky, S. L., Prueksaritanont, T., Ramjit, H. G., Slaughter, D. E., and Vyas, K. P. (1999) In vitro and in vivo studies on the metabolism of tirofiban. Drug Metab Dispos 27, 1360-1366) are clinically approved in the United States. Abciximab, the first approved and most widely used agent, is a chimeric Fab with mouse variable and human constant domains. It binds to an epitope adjacent to the ligand binding region and inhibits fibrinogen binding by steric hindrance. Abciximab was reported to cross-react with integrin αvβ3 and αMβ2(Scarborough, R. M. (1999) Development of eptifibatide. Am Heart J 138, 1093-1104). Eptitibatide and tirofiban, on the other hand, are small molecule drugs that bind to the RGD ligand interaction site of the integrin and are αIIbβ3-specific. They showed lower affinity and much shorter circulatory half-lives than abciximab (Scarborough, R. M. (1999) Development of eptifibatide. Am Heart J 138, 1093-1104; Vickers, S., Theoharides, A. D., Arison, B., Balani, S. K., Cui, D., Duncan, C. A., Ellis, J. D., Gorham, L. M., Polsky, S. L., Prueksaritanont, T., Ramjit, H. G., Slaughter, D. E., and Vyas, K. P. (1999) In vitro and in vivo studies on the metabolism of tirofiban. Drug Metab Dispos 27, 1360-1366; McClellan, K. J., and Goa, K. L. (1998) Tirofiban. A review of its use in acute coronary syndromes. Drugs 56, 1067-1080).
Acute thrombocytopenia is a recognized complication of treatment with integrin αIIbβ3 inhibitors. Thrombocytopenia, often severe (platelets less than 50×109/L), occurs in 0.5% to 1% of patients given abciximab for the first time (Berkowitz, S. D., Sane, D. C., Sigmon, K. N., Shavender, J. H., Harrington, R. A., Tcheng, J. E., Topol, E. J., and Califf, R. M. (1998) Occurrence and clinical significance of thrombocytopenia in a population undergoing high-risk percutaneous coronary revascularization. Evaluation of c7E3 for the Prevention of Ischemic Complications (EPIC) Study Group. J Am Coll Cardiol 32, 311-319; Jubelirer, S. J., Koenig, B. A., and Bates, M. C. (1999) Acute profound thrombocytopenia following C7E3 Fab (Abciximab) therapy: case reports, review of the literature and implications for therapy. Am J Hematol 61, 205-208; Kereiakes, D. J., Berkowitz, S. D., Lincoff, A. M., Tcheng, J. E., Wolski, K., Achenbach, R., Melsheimer, R., Anderson, K., Califf, R. M., and Topol, E. J. (2000) Clinical correlates and course of thrombocytopenia during percutaneous coronary intervention in the era of abciximab platelet glycoprotein IIb/IIIa blockade. Am Heart J 140, 74-80) and in 4% of patients after the second administration (Madan, M., Kereiakes, D. J., Hermiller, J. B., Rund, M. M., Tudor, G., Anderson, L., McDonald, M. B., Berkowitz, S. D., Sketch, M. H., Jr., Phillips, H. R., 3rd, and Tcheng, J. E. (2000) Efficacy of abciximab readministration in coronary intervention. Am J Cardiol 85, 435-440; Tcheng, J. E., Kereiakes, D. J., Braden, G. A., Jordan, R. E., Mascelli, M. A., Langrall, M. A., and Effron, M. B. (1999) Readministration of abciximab: interim report of the ReoPro readministration registry. Am Heart J 138, S33-38). In clinical trials of tirofiban the incidence ranged from 0.1% to 0.5%, which is only twice the incidence seen in patients not given the drug (The RESTORE Investigators. Randomized Efficacy Study of Tirofiban for Outcomes and REstenosis (1997) Effects of platelet glycoprotein IIb/IIIa blockade with tirofiban on adverse cardiac events in patients with unstable angina or acute myocardial infarction undergoing coronary angioplasty. Circulation 96, 1445-1453; Platelet Receptor Inhibition in Ischemic Syndrome Management (PRISM) Study Investigators. (1998) A comparison of aspirin plus tirofiban with aspirin plus heparin for unstable angina. N Engl J Med 338, 1498-1505; Platelet Receptor Inhibition in Ischemic Syndrome Management in Patients Limited by Unstable Signs and Symptoms (PRISM-PLUS) Study Investigators. (1998) Inhibition of the platelet glycoprotein IIb/IIIa receptor with tirofiban in unstable angina and non-Q-wave myocardial infarction. N Engl J Med 338, 1488-1497). In the PURSUIT trial of eptifibatide, the incidence was approximately the same in patients given the study drug as in patients given placebo (McClure, M. W, Berkowitz, S. D., Sparapani, R., Tuttle, R., Kleiman, N. S., Berdan, L. G., Lincoff, A. M., Deckers, J., Diaz, R., Karsch, K. R., Gretler, D., Kitt, M., Simoons, M., Topol, E. J., Califf, R. M., and Harrington, R. A. (1999) Clinical significance of thrombocytopenia during a non-ST-elevation acute coronary syndrome. The platelet glycoprotein IIb/IIIa in unstable angina: receptor suppression using integrilin therapy (PURSUIT) trial experience. Circulation 99, 2892-2900). Only a small subset of patients given eptifibatide had profound, unexplained thrombocytopenia (McClure, M. W, Berkowitz, S. D., Sparapani, R., Tuttle, R., Kleiman, N. S., Berdan, L. G., Lincoff, A. M., Deckers, J., Diaz, R., Karsch, K. R., Gretler, D., Kitt, M., Simoons, M., Topol, E. J., Califf, R. M., and Harrington, R. A. (1999) Clinical significance of thrombocytopenia during a non-ST-elevation acute coronary syndrome. The platelet glycoprotein IIb/IIIa in unstable angina: receptor suppression using integrilin therapy (PURSUIT) trial experience. Circulation 99, 2892-2900). The cause of thrombocytopenia induced by integrin αIIbβ3 inhibitors is not known, although recently it was reported that human antibodies against the mouse variable domains of abciximab, presumably induced by the first exposure of abciximab, were the cause of platelet destruction in patients who developed severe thrombocytopenia after being given abciximab a second time (Curtis, B. R., Swyers, J., Divgi, A., McFarland, J. G., and Aster, R. H. (2002) Thrombocytopenia after second exposure to abciximab is caused by antibodies that recognize abciximab-coated platelets. Blood 99, 2054-2059).
Integrins αvβ3, αvβ5, αIIbβ3, and α5β1 bind to a variety of adhesive protein containing an Arg-Gly-Asp (RGD) tripeptide. Exploiting this feature, adhesive protein-mimicking synthetic human mAbs containing an RGD motif in HCDR3 flanked by six randomized residues were selected from antibody libraries by phage display (Barbas, C. F., 3rd, Languino, L. R., and Smith, J. W. (1993) High-affinity self-reactive human antibodies by design and selection: targeting the integrin ligand binding site. Proc Natl Acad Sci USA 90, 10003-10007). Fab-9, the most potent of these antibodies, binds to integrin αvβ3 nearly 1,000-fold better than to integrins αvβ5 and α5β1. However, neither of the selected antibodies including Fab-9 distinguished between the two β3 integrins, αvβ3 and αIIbβ3. Therefore, a motif optimization library was generated to determine whether further rounds of engineering and selection on the RGD motif within the HCDR3 of Fab-9 could produce an antibody with specificity for either integrin αvβ3 or αIIbβ3 (Smith, J. W., Hu, D., Satterthwait, A., Pinz-Sweeney, S., and Barbas, C. F., 3rd (1994) Building synthetic antibodies as adhesive ligands for integrins. J Biol Chem 269, 32788-32795). While many of the selected antibodies showed some preference to either integrin αvβ3 or integrin αIIbβ3, all antibodies still bound to both integrins. The majority of the selected antibodies had the consensus sequence (K/R)XD. Interestingly, the middle position in the RGD motif proved to be highly permissive. It was found that the Gly can be substituted by Val, Ala, Asn, Arg, Thr, Gln, Asp, Ser, and Trp (Smith, J. W., Hu, D., Satterthwait, A., Pinz-Sweeney, S., and Barbas, C. F., 3rd (1994) Building synthetic antibodies as adhesive ligands for integrins. J Biol Chem 269, 32788-32795).